The present invention relates to a sealing and guide unit for pistons in general.
As far as pistons and in particular the necessity to ensure a dynamic seal between surfaces with an alternate relative motion are concerned, many variously shaped and combined sealing elements have been proposed, also to ensure a correct alignment and guide between the relevant surfaces.
It has however been noticed that if the packing means are elastic enough to ensure a tight sealing they cannot be rigid as required for a safe guide or support of a surface, e.g. that of a piston, in respect to another surface, e.g. that of the cylinder for the piston.
In fact, when using any sealing elements made of a sufficiently elastic material to ensure a tight seal and at least a certain aligning or supporting effect, there is always an excessive and wearing friction owing to specific stresses and causing a rapid deterioration of the whole unit. If instead the materials employed have the rigidity required for a correct guide or support, the seal will never be tight enough to prevent substantial and unbearable leaks due to excessive tolerances.
As a matter of fact, some trials have already been made to solve the problem of a perfectly tight seal also acting as a guide or support, but so far the results achieved have not been sufficient to comply with the known requests.
One of the solutions proposed to this end is a sealing and guide system comprising a ring of elastic material to ensure static sealing which is peripherally coupled with a ring of antifriction material acting as a dynamic sliding seal. This assembly features a fair elastic reaction allowing the self-alignment of the piston, but on the other hand it is not able to ensure a sufficient guiding support, in particular when the piston is subject to radial thrusts, while said thrusts are usually present in this kind of assemblies. That is why, to ensure a correct support, at least a second rigid ring of antifriction material is added and placed in parallel to the sealing unit composed of the closely coupled elastic and rigid rings. The additional guide ring is usually a split ring fitted into a groove made available for this purpose. In this way the original composite unit is substantially a sealing unit, the additional ring acting as a supporting and guiding element for the piston inside its chamber.
The above described structure, however, requires higher production costs and also increases the dimensions of the piston and those of the whole assembly.
Several other solutions have also been proposed, using changes in shape, dimensions and sections, but none of them has so far achieved any really satisfactory results.